This invention relates in general to surface inspection systems, and in particular, to a high speed scanner system for inspecting anomalies (contaminant particles and pattern defects) on surfaces. In particular, it relates to a surface inspection system for inspecting anomalies on substantially planar surfaces such as those of semiconductor wafers, photomasks, reticles, ceramic tiles, and other surfaces.
The size of semiconductor devices fabricated on silicon wafers has been continually reduced. At the time that this application is filed, for example, semiconductor devices can be fabricated at a resolution of a half micron or less, and sixty-four (64) megabits DRAMs are being fabricated with a 0.35 micron design rule. The shrinking of semiconductor devices to smaller and smaller sizes has imposed a much more stringent requirement on the sensitivity of wafer inspection instruments which are called upon to detect contaminant particles and pattern defects that are small compared to the size of the semiconductor devices. At the same time, it is desirable for wafer inspection systems to provide an adequate throughput so that these systems can be used for in-line inspection to detect wafer defects.
In U.S. Pat. No. 4,898,471 to Stonestrom et al. assigned to the present assignee of this application, the area illuminated on a wafer surface by a scanning beam is an ellipse which moves along a scan line henceforth called a sweep. In one example given by Stonestrom et al., the ellipse has a width of 20 microns and a length of 115 microns. Light scattered by anomalies or patterns in such illuminated area is detected by photodetectors placed at azimuthal angles in the range of 80.degree. to 100.degree.. The signals detected by the photodetectors are used to construct templates. When the elliptical spot is moved along the scan line to a neighboring region, scattered light from structures within the spot is again detected and the photodetector signal is then compared to a template to ascertain the presence of contaminant particles or pattern defects as opposed to regular pattern. In Stonestrom et al., the scanning beam scans across the entire wafer during each sweep, illuminating and inspecting the wafer, while the wafer is simultaneously moved by a mechanical stage in a direction substantially perpendicular to the sweep direction. This operation is then repeated until the entire wafer has been inspected.
While the system of Stonestrom et al. performs well for inspecting wafers having semiconductor devices that are fabricated with coarser resolution, with the continual shrinking of the size of the devices fabricated, it is now desirable to provide an improved inspection tool that can be used to detect very small anomalies that may be difficult to detect using Stonestrom et al.'s system.
Another type of surface inspection system is an imaging system that illuminates a large area and scans duplicate areas of surfaces, such as those of photomasks or semiconductor wafers. Optically scanned information from the duplicate areas is then compared to determine differences therebetween. As examples of such systems, see U.S. Pat. Nos. 4,532,650; 4,579,455; 4,805,123 and 4,926,489. In such systems, where the pixel is submicron in size, the optical scanning system requires significant time to scan the entire surface of a photomask or semiconductor wafer so that the throughput of such systems is typically low and usually not suitable for in-line inspection.
It is therefore desirable to provide an improved surface inspection system with adequate sensitivity for detecting small particles while achieving an adequate throughput at reasonable cost so that these systems can be used for in-line inspection to detect wafer defects.